1. Technical Field
This invention relates to the removal of heavy metal ions from waste and process streams. More particularly, it relates to the precipitation and removal from waste streams of iron, copper and nickel ions which have become bonded to organic complexing agents.
2. Background Art
Steam boilers from power plants need to have periodic internal cleaning and removal of heavy metal oxide or rust sediment to prevent corrosion and maintain efficiency. The cleaning solutions are composed of powerful organic complexing agents such as ethylenediaminetetraacetic acid, citric acid, etc. These complexing agents clean the boilers and in particular remove the metal oxide rust and scales by water soluble complex formation. Aqueous waste from the boiler cleaning now, however, contains heavy metal ions, expecially, iron, copper and nickel, in complexed form and it becomes nearly impossible to precipitate out last traces of these metal ions by expected conventional means such as increasing the pH of the waste solution with sodium or calcium hydroxides. Environmental regulations, depending on the locale, can require that iron, copper and nickel ion content of waste solution be reduced from a few mg/l to a fraction of a mg/l of the waste solution before any waste can be discharged into a receiving stream. Present methods of disposal of such heavy metal containing wastes are expensive and troublesome such as contract disposal or the hauling of the waste to a site where approved secondary and tertiary treatment facilities are available.
This invention provides a relatively inexpensive and efficient process using environmentally acceptable chemicals to remove the iron, copper and nickel ions from an aqueous waste or process stream.